The present invention relates to the field of computed radiography. More particularly, it relates to improvements in apparatus for recovering the image information contained by a previously exposed radiographic recording element comprising a stimulable phosphor.
U.S. Pat. No. 3,859,529 (Re. No. 31,847) to G. W. Luckey discloses the basic concept of using stimulable phosphors to record x-ray patterns and of recovering (reading-out) such patterns by scanning such phosphors with a beam of radiation adapted to stimulate luminescence (fluorescence or phosphorescence) from the x-ray exposed portions of the phosphor. During the scan-stimulation process, the phosphor luminescence is detected by a photodetector (e.g., a photomultiplier tube) to provide an electrical signal representative of the recorded x-ray pattern. Such signal may, if desired, be digitized and processed, through suitable algorithms, to reduce noise, enhance edges, increase contrast, etc. The processed electrical signal may then be used, for example, to intensity-modulate a scanning laser beam to record the x-ray pattern on a photosensitive film or the like.
In using the Luckey process for the purpose of diagnostic radiography (e.g., for mammography) relatively low levels of x-ray radiation are used to record the desired x-ray pattern. Since the stimulated luminescence intensity is proportional to the intensity of x-radiation received by the image-storing phosphor, it will be appreciated that the stimulated luminescence level can be very low, especially for such diagnostic radiographic applications. To produce useful images, a major portion of the stimulated luminescence must be collected for use in producing the image signal.
U.S. Pat. No. 4,346,295 issued to Tanaka et al. discloses an optical arrangement for efficiently collecting a maJor portion of the luminescence given off by the image-storing phosphor during the scan-stimulation step. In the Tanaka et al. reference, a laser beam scans the phosphor, point by point, along a rectilinear image line. A light guide member, made of a transparent sheet having smooth surfaces for effecting total internal reflection, is arranged so that a flat, linear surface thereof is closely spaced with respect to the scan line on the stimulable phosphor. Such flat surface is typically 5-8 mm in width and is sufficiently long to span the phosphor plate (about 100-400 mm). As the scanning laser beam stimulates the phosphor, the stimulated luminescence enters the light guide through the flat end thereof and is internally reflected toward a distal end which is curled to form an annulus. A photomultiplier tube is optically coupled to the annular end of the light guide to detect the luminescence transmitted by the light guide.
While the luminescence-detecting apparatus disclosed by Tanaka et al is relatively efficient at collecting a large percentage of the photons constituting the luminescence given off by the stimulated phosphor, a significant percentage of these photons are reflected by the flat surface of the light-guide member and, thus, are not detected by the photomultiplier tube. More significantly, however, is the fact that the stimulating radiation, which can be 10.sup.8 times more intense than the stimulated luminescence, also gets reflected by the flat surface of the light-guide. A major portion of this reflected stimulating radiation is returned to the phosphor plate and acts to prematurely stimulate luminescence from non-addressed regions of the plate, i.e., those regions not directly irradiated by the scanning laser beam. Such reflected stimulating radiation, known as "flare," has the adverse effects of adding background noise to the system, thereby reducing the signal-to-noise ratio of the read-out signal. Moreover, it gives rise to false signals and ghost images which substantially degrade the quality of the ultimate image.